(1) Field of the Invention
The present invention relates to an air inlet for an engine and more particularly for a turbine engine of a rotary wing aircraft. This air inlet is provided with a filter system combining at least one barrier filter with at least one bypass device. The technical field of the invention is thus the field of turbine engine air inlets, and more particularly the field of filter systems for such air inlets.
(2) Description of Related Art
In order to feed each engine of an aircraft with outside air and thus enable the engine(s) to operate, the aircraft has at least one air inlet, each air inlet being provided with a filter system and with a duct connecting the engine(s) to the outside air.
Furthermore, each air inlet may be positioned in various locations relative to the engine and to the forward direction of the aircraft. More particularly, two types of air inlet may be distinguished.
Firstly, a longitudinal or axial air inlet can be placed facing the main advance direction of the aircraft. Such an inlet is also said to be “dynamic” since it is fed with outside air under the effects both of the forward speed of the aircraft and of the suction from the engine; and
Secondly, a lateral or radial air inlet can be placed perpendicularly to the main forward direction of the aircraft, i.e. in general in the side or in the top of the fuselage of the aircraft. Such an inlet is also said to be “static”, since it is fed with air solely under the effect of suction from the engine.
Aircraft can be required to operate in a wide variety of environments and conditions, and each engine of such aircraft must therefore be protected in order to be able to withstand such environments and such conditions.
Firstly, the wash from the lift rotor of a rotary wing aircraft often raises dust during takeoff and landing, together with gravel lying on the ground. It is therefore essential to protect the air inlet of each engine to avoid it ingesting any raised elements. It is also necessary to protect each engine from various particles in suspension in the air, in particular when the aircraft is used in sandy regions where the air can be saturated with particles of sand.
For this purpose, each air inlet of the aircraft is provided with at least one filter system for stopping such particles and thus protecting each engine. Such filter systems make use of various types of filter, such as barrier filters with one or more paper or fabric layers to stop particles in suspension or vortex filters that create a vortex for the purpose of separating particles from the ingested air.
Secondly, aircraft may be required to fly in so-called “icing” conditions. During this type of flight, ice can obstruct the air inlet of an engine in part or even in full, in particular by ice becoming attached to the barrier filter. Thus, each filter may become partially or even completely obstructed by ice.
Similarly, various particles such as dust or sand can obstruct each filter of the filter system in part or in full, in particular when using barrier filters, thereby limiting or even preventing outside air from passing to the engine.
Consequently, the feed of outside air to the engine is reduced or even interrupted, thereby giving rise to a significant or even total loss of the power developed by the engine, which can lead to an accident.
Consequently, in order to guarantee that each engine of the aircraft can operate, and in particular in order to guarantee that it is fed with outside air, regardless of the outside conditions and the environment of the aircraft and regardless of the state of the air inlet filter, an aircraft maker can provide a passage for feeding outside air to the engine while avoiding the filter. Such a device ensures flight is safe when flying in an atmosphere that is, for example, loaded with particles or with ice. Indeed, such a device is required by certain aviation certifications. Such a device is commonly referred to as a “bypass device” since it is used when filter is obstructed.
Document U.S. Pat. No. 3,411,272 describes a filter system for a helicopter engine, the system having a closed position for filtering air and an open position for allowing air to pass without being filtered, i.e. a bypass position. That filter system has filtering louvers that touch one another in the closed position so as to filter air. Some of the louvers are movable and pivot in order to open the filter system in the bypass position, where necessary.
Document WO 2008/076471 also describes a filter system for an aircraft engine, the filter system possessing one or more filter panels flush with the outside surface of the aircraft. Each filter panel is connected to an actuator enabling the entire filter panel to be pivoted and thus enabling air to pass without passing through the filter panel, thereby creating a bypass position. Nevertheless, that filter system moves the entire filter panel, which may be bulky and heavy, thus requiring an actuator that is powerful.
Document U.S. Pat. No. 5,662,292 describes a filter system for a helicopter engine. That filter system has filter zones in front and on the sides relative to the longitudinal direction of the helicopter, thereby allowing air to enter equally well while the helicopter is flying forwards and while it is flying sideways. That filter system includes a flap situated on the top of the filter and suitable for tilting under the effect of a spring into a bypass position, thereby releasing an opening that allows air to pass without passing through the filter zones.
Finally, document EP 2282031 describes a filter system that incorporates a bypass device. That filter system is adapted for a radial air inlet of an engine and comprises an annular filter having a stationary portion and a movable portion. The movable portion of the annular filter pivots about the axis of the engine and of the annular filter, and thus makes it possible to release the air inlet of the annular filter, thereby creating a bypass position.
Other filter systems exist and some of them have two different air flow ducts, one duct feeding the engine with filtered air and the other duct being specific to the bypass device. However, such filter systems can be quite complex and heavy. Furthermore, the use of two different feed ducts can give rise in the air inlet to differences of temperature and pressure in the air being fed to the engine, thereby giving rise to variations in the performance of the engine. In addition, during a flight, it is possible that dust, sand, or debris can accumulate in the specific flow duct of the bypass device and subsequently be sucked into the engine when the bypass device is put into operation.